Multi-scale structures of the mammalian radial spoke and divergence of axonemal complexes in ependymal cilia

Trypanosome doublet microtubule structures reveal flagellum assembly and motility mechanisms

The flagellum of Trypanosoma brucei drives the parasite's characteristic screw-like motion and is essential for its replication, transmission, and pathogenesis. However, the molecular details of this process remain unclear. Here, we present high-resolution (up to 2.8 angstrom) cryo-electron microscopy structures of T. brucei flagellar doublet microtubules (DMTs). Integrated modeling identified 154 different axonemal proteins inside and outside the DMT and, together with genetic and proteomic interrogation, revealed conserved and trypanosome-specific foundations of flagellum assembly and motility. We captured axonemal dynein motors in their pre-power stroke state. Comparing atomic models between pre- and post-power strokes defined how dynein structural changes drive sliding of adjacent DMTs during flagellar beating. This study illuminates structural dynamics underlying flagellar motility and identifies pathogen-specific proteins to consider for therapeutic interventions targeting neglected diseases.

IQUB mutation induces radial spoke 1 deficiency causing asthenozoospermia with normal sperm morphology in humans and mice

BACKGROUND

Asthenozoospermia (ASZ) accounts for about 20-40% of male infertility, and genetic factors, contributing to 30-40% of the causes of ASZ, still need further exploration. Radial spokes (RSs), a T-shaped macromolecular complex, connect the peripheral doublet microtubules (DMTs) to a central pair (CP), forming a CP-RS-DMT structure to regulate the beat frequency and amplitude of sperm flagella. To date, many components of RSs and their functions in human sperm flagella remain unclear.

METHODS

We recruited a cohort of 323 infertile males with ASZ between August 2019 and June 2024. Genetic mutations were identified by whole-exome sequencing. Computer-aided sperm analysis, Papanicolaou staining, and electron microscopy were applied to evaluate the motility, morphology, and ultrastructure of spermatozoa, respectively. Protein mass spectrometry, western blotting, and bioinformatic analyses were performed to identify critical components of mammalian RS1 to model its structure and explore the pathological mechanism of IQUB deficiency. Intracytoplasmic sperm injection (ICSI) was applied for the patient and Iqub-/- mice.

RESULTS

We identified a novel homozygous IQUB mutation [c.842del (p.L281Pfs*28)] in an ASZ male with normal sperm morphology (ANM), which resulted in the complete loss of IQUB in sperm flagella. Deficiency of RS1, but not RS2 or RS3, was observed in both IQUB842del patient and Iqub-/- mice, and resulted in the reduction of sperm kinetic parameters, indicating the critical role of IQUB in regulating mammalian RS1 assembly and sperm flagellar beat. More importantly, we identified twelve critical components of RS1 in humans and mice, among which RSPH3, RSPH6A, RSPH9 and DYDC1 constituting the head, DYDC1, NME5, DNAJB13 and PPIL6 assembling into the head-neck complex, AK8, ROPN1L, RSPH14, DYNLL1, and IQUB forming the stalk of RS1. Along with the RS1 defect, the IQUB deficiency caused significant down-regulation of the inner dynein arms of DNAH7 and DNAH12, highlighting their nearby location with RS1. Finally, ICSI can effectively resolve the male infertility caused by IQUB genetic defects.

CONCLUSIONS

We demonstrate that IQUB may serve as an adapter for sperm flagellar RS1 in both humans and mice and consolidated the causal relationship between IQUB genetic mutations and ANM, further enriching the genetic spectrum of male infertility.

Structural diversity of axonemes across mammalian motile cilia

Reproduction, development and homeostasis depend on motile cilia, whose rhythmic beating is powered by a microtubule-based molecular machine called the axoneme. Although an atomic model of the axoneme is available for the alga Chlamydomonas reinhardtii1, structures of mammalian axonemes are incomplete1-5. Furthermore, we do not fully understand how molecular structures of axonemes vary across motile-ciliated cell types in the body. Here we use cryoelectron microscopy, cryoelectron tomography and proteomics to resolve the 96-nm modular repeat of axonemal doublet microtubules (DMTs) from both sperm flagella and epithelial cilia of the oviduct, brain ventricles and respiratory tract. We find that sperm DMTs are the most specialized, with epithelial cilia having only minor differences across tissues. We build a model of the mammalian sperm DMT, defining the positions and interactions of 181 proteins including 34 newly identified proteins. We elucidate the composition of radial spoke 3 and uncover binding sites of kinases associated with regeneration of ATP and regulation of ciliary motility. We discover a sperm-specific, axoneme-tethered T-complex protein ring complex (TRiC) chaperone that may contribute to construction or maintenance of the long flagella of mammalian sperm. We resolve axonemal dyneins in their prestroke states, illuminating conformational changes that occur during ciliary movement. Our results illustrate how elements of chemical and mechanical regulation are embedded within the axoneme, providing valuable resources for understanding the aetiology of ciliopathy and infertility, and exemplifying the discovery power of modern structural biology.

CCDC113 stabilizes sperm axoneme and head-tail coupling apparatus to ensure male fertility

The structural integrity of the sperm is crucial for male fertility, defects in sperm head-tail linkage and flagellar axoneme are associated with acephalic spermatozoa syndrome (ASS) and the multiple morphological abnormalities of the sperm flagella (MMAF). Notably, impaired head-tail coupling apparatus (HTCA) often accompanies defects in the flagellum structure, however, the molecular mechanisms underlying this phenomenon remain elusive. Here, we identified an evolutionarily conserved coiled-coil domain-containing (CCDC) protein, CCDC113, and found the disruption of CCDC113 produced spermatozoa with disorganized sperm flagella and HTCA, which caused male infertility. Further analysis revealed that CCDC113 could bind to CFAP57 and CFAP91, and function as an adaptor protein for the connection of radial spokes, nexin-dynein regulatory complex (N-DRC), and doublet microtubules (DMTs) in the sperm axoneme. Moreover, CCDC113 was identified as a structural component of HTCA, collaborating with SUN5 and CENTLEIN to connect sperm head to tail during spermiogenesis. Together, our studies reveal that CCDC113 serve as a critical hub for sperm axoneme and HTCA stabilization in mice, providing insights into the potential pathogenesis of infertility associated with human CCDC113 mutations.

Cryo-EM structure of PML RBCC dimer reveals CC-mediated octopus-like nuclear body assembly mechanism

Promyelocytic leukemia protein (PML) nuclear bodies (NBs) are essential in regulating tumor suppression, antiviral response, inflammation, metabolism, aging, and other important life processes. The re-assembly of PML NBs might lead to an ~100% cure of acute promyelocytic leukemia. However, until now, the molecular mechanism underpinning PML NB biogenesis remains elusive due to the lack of structural information. In this study, we present the cryo-electron microscopy (cryo-EM) structure of the PML dimer at an overall resolution of 5.3 Å, encompassing the RING, B-box1/2 and part of the coiled-coil (RBCC) domains. The integrated approach, combining crosslinking and mass spectrometry (XL-MS) and functional analyses, enabled us to observe a unique folding event within the RBCC domains. The RING and B-box1/2 domains fold around the α3 helix, and the α6 helix serves as a pivotal interface for PML dimerization. More importantly, further characterizations of the cryo-EM structure in conjugation with AlphaFold2 prediction, XL-MS, and NB formation assays, help unveil an unprecedented octopus-like mechanism in NB assembly, wherein each CC helix of a PML dimer (PML dimer A) interacts with a CC helix from a neighboring PML dimer (PML dimer B) in an anti-parallel configuration, ultimately leading to the formation of a 2 µm membrane-less subcellular organelle.

The mechanics of cilia and flagella: What we know and what we need to know

In this review, we provide a condensed overview of what is currently known about the mechanical functioning of the flagellar/ciliary axoneme. We also present a list of 10 specific areas where our current knowledge is incomplete and explain the benefits of further experimental investigation. Many of the physical parameters of the axoneme and its component parts have not been determined. This limits our ability to understand how the axoneme structure contributes to its functioning in several regards. It restricts our ability to understand how the mechanics of the structure contribute to the regulation of motor function. It also confines our ability to understand the three-dimensional workings of the axoneme and how various beating modes are accomplished. Lastly, it prevents accurate computational modeling of the axoneme in three-dimensions.

The Molecular Basis of Multiple Morphological Abnormalities of Sperm Flagella and Its Impact on Clinical Practice

Multiple morphological abnormalities of the sperm flagella (MMAF) is a specific form of severe flagellar or ciliary deficiency syndrome. MMAF is characterized by primary infertility with abnormal morphology in the flagella of spermatozoa, presenting with short, absent, bent, coiled, and irregular flagella. As a rare disease first named in 2014, studies in recent years have shed light on the molecular defects of MMAF that comprise the structure and biological function of the sperm flagella. Understanding the molecular genetics of MMAF may provide opportunities for the development of diagnostic and therapeutic strategies for this rare disease. This review aims to summarize current studies regarding the molecular pathogenesis of MMAF and describe strategies of genetic counseling, clinical diagnosis, and therapy for MMAF.

Loss of an extensive ciliary connectome induces proteostasis and cell fate switching in a severe motile ciliopathy

Motile cilia have essential cellular functions in development, reproduction, and homeostasis. Genetic causes for motile ciliopathies have been identified, but the consequences on cellular functions beyond impaired motility remain unknown. Variants in CCDC39 and CCDC40 cause severe disease not explained by loss of motility. Using human cells with pathological variants in these genes, Chlamydomonas genetics, cryo-electron microscopy, single cell RNA transcriptomics, and proteomics, we identified perturbations in multiple cilia-independent pathways. Absence of the axonemal CCDC39/CCDC40 heterodimer results in loss of a connectome of over 90 proteins. The undocked connectome activates cell quality control pathways, switches multiciliated cell fate, impairs microtubule architecture, and creates a defective periciliary barrier. Both cilia-dependent and independent defects are likely responsible for the disease severity. Our findings provide a foundation for reconsidering the broad cellular impact of pathologic variants in ciliopathies and suggest new directions for therapies.